Numerous tasks, for example cutting sheet metal or abrading a surface, may be accomplished through the use of a stream of pressurized fluid, typically water, which is generated by high-pressure, positive displacement pumps. Such pumps pressurize a fluid by having a reciprocating plunger that draws the fluid from an inlet area into a pressurization chamber during an intake stroke, and acts against the fluid during a pumping stroke, thereby forcing pressurized fluid to pass from the pressurization chamber through a passageway to an outlet check valve which selectively allows the pressurized fluid to pass into an outlet chamber. The pressurized fluid in the outlet chamber is then collected in a manifold to be used by an operator via whatever tool has been attached to the pump for a particular task.
During the normal course of operation, the required flow rate will vary from the maximum the pump can supply to zero, for example, when the operator turns the tool off. In this situation, where the pressurized fluid is not being used, the pressure in the outlet chamber will build up beyond an acceptable level unless some form of pressure control is incorporated into the pump. If no pressure control is provided, the buildup of high pressure will result in damage and stress to the parts of the pump and undesirable surges of pressure will occur when the operator again turns the tool on.
One method of pressure control which is currently used is to incorporate a relief valve into the pump. When the pressure in the outlet chamber rises above a preset limit as a result of pressurizing more water than is demanded by the end user, the relief valve opens to vent the excess pressurized fluid. This method has several disadvantages, however. Perhaps most significantly, it is very expensive and inefficient to pressurize water thereby generating potential energy, only to throw it away. This throwing away of energy results in increased maintenance and fuel costs. This method of controlling output pressure is also undesirable because of the large quantity of water that is thrown away as waste, rather than being used.
Another method considered in the course of developing the present invention for controlling the output pressure of the pump, which is substantially equivalent to the pressure in the outlet chamber, is to choke off the flow at the inlet. However, this method causes the fluid to cavitate, which results in significant damage to the pump. Such damage in turn increases the "down time" of the machine and increases cost of operation, both in labor and replacement parts. This method also causes the system to have a large time constant, which results in undesirable pressure oscillations.